1. Field of the Invention
The present invention relates to a diffractive optical element having high diffraction efficiency at a plurality of wavelengths and an image-pickup optical system using the diffractive optical element.
2. Description of the Related Art
Previously, as a method of reducing a chromatic aberration of an optical system, a method of providing a diffractive optical element having a diffractive action as a part of an image-pickup optical system has been known. It is preferred that the diffractive optical element be manufactured by a thermal press forming of a glass in view of cost, but there is a problem that an end of a grating is cracked when forming the grating of the diffractive optical element. Therefore, practically, it is limited to a single-layer diffractive optical element that has a low grating.
A contact-type diffractive optical element can maintain high diffraction efficiency over a wide wavelength range, but a grating height of the diffractive grating has a height from a few micrometers to a few dozens of micrometers. Therefore, in order to form the grating of the glass, a method of easily separating the glass from the mold or a countermeasure of the grating crack is needed. Japanese Patent Laid-Open No. 2000-302463 discloses a method of manufacturing a diffractive optical element using the thermal press forming of a glass in which a peripheral part that has no grating is provided with a step and also the step is tapered to provoke the separation of the glass and a mold. Japanese Patent Laid-Open No. 2002-62416 discloses a configuration that prevents a glass from entering the deepest part of a grating portion of a mold to ensure the separation of the glass and the mold. Japanese Patent Laid-Open No. 2004-157404 discloses a diffractive optical element that has a grating whose end is round to prevent a grating crack.
However, in the method of Japanese Patent Laid-Open No. 2000-302463, the phenomenon that the end of the grating is hooked on the mold cannot be prevented although the separation is provoked in the peripheral part. In the configurations of Japanese Patent Laid-Open No. 2002-62416 and Japanese Patent Laid-Open No. 2004-157404, the shape of the grating end of the diffractive grating is cracked, and it is difficult to maintain high diffraction efficiency in a wide wavelength range. Thus, in the conventional diffractive optical element, the generation of unnecessary light cannot be suppressed.